The present invention relates to a scanning electron microscope such as a scanning ion microscope or a scanning electron microscope for scanning a specimen surface with charged particles to acquire an image and, more particularly, to a scanning electron microscope capable improving a resolving power and an S/N ratio by processing an acquired image and to an image restoring method for restoring the image acquired by the scanning electron microscope.
In order to observe a fine object clearly, there is widely utilized a scanning electron microscope having an extremely higher resolving power than that of an optical microscope. The scanning electron microscope acquires the information of a target specimen at a beam irradiated position by irradiating the target specimen with a focused electron beam and by detecting the charged particles (as may belong to a kind different from that of the irradiating charged particles) emitted from the specimen or transmitting the specimen. An enlarged image of the target specimen can be acquired by scanning the specimen with the charged particle beam.
The scanning ion microscope (as will be abbreviated into “SIM”) or the scanning electron microscope (as will be abbreviated into “SEM”) is well known as one scanning charged particle microscope. Especially in a semiconductor manufacturing process, the scanning charged particle microscope is used in an application not only for observing the image but also for determining the featuring quantity of the target specimen to inspect the semiconductor or to measure the pattern, thereby to detect the defect having occurred on the semiconductor wafer and inspect the cause for the defect and to measure the size and shape of the pattern. As the pattern has become the more minute, the necessity for inspecting the minute defect or for measuring the pattern highly precisely becomes the higher, and it becomes important to acquire an image of a high resolving power.
However, even any scanning charged particle microscope has a limit in the resolving power. Due to the diffraction aberration to be caused by the wave motion property of the particle and the chromatic aberration and the spherical aberration to be caused by the characteristics of a lens, the charged particle beam (such as an ion beam or an electron beam) is incident on the specimen surface with the intensity distribution of the beam diverged by those aberrations. Moreover, the charged particle beam incident in the specimen is generally diffused in the specimen, and is then emitted from the specimen or passes through the specimen. These phenomena cause the deterioration of the resolving power. In the scanning charged particle microscope, moreover, as the more charged particle beams are incident, there arises the more serious problem that the specimen is damaged or that the imaging time period becomes long. As a result, the charged particle beam of a sufficient quantity cannot be incident, and the signal quantity detected is decreased to make the S/N ratio of the taken image lower in the scanning charged particle microscope than in the optical microscope.
On the other hand, an image restoration is known as the image processing for making it possible to improve the resolution and the S/N ratio of the image. The image, as obtained by a device such as a camera, a telescope or a microscope, never fails to have a deterioration of the resolving power and a superposition of noises. The image restoration is a process for estimating a clear image of a high S/N ratio (as will be called the “ideal image”), which is prepared by eliminating the deterioration of the resolving power and the noises from those images. In the ordinary image restoration, the image, which is deteriorated in the resolving power by convoluting the point spread function indicating the resolving power deterioration degree of the ideal image and in which the image having the superposed noises is modeled as the taken image, and this model is used to estimate the ideal image reversely from the taken image. In connection with the image restoration, many researches have been performed on an astronomical image or an optical image (as referred to A. K. Katsaggelos: Optical Engineering, 28, 7, pp. 735-748 (1989), or M. R. Banham and A. K. Katsaggelos: IEEE Signal Processing Magazine, pp. 24-41 (March 1997), for example).
In the scanning optical microscope or the scanning charged particle microscope, too, a method for improving the resolving power by using the image restoration is proposed in JP-A-3-44613 or Y. I. Gold and A. Goldenshtein: Proc. SPIE, 3332, pp. 620-624 (1998).
Moreover, a calculating method for determining the beam intensity distribution on the specimen surface or one resolving power deterioration factor of the taken image is described in J. Orloff: Handbook of Charged Particle Optics, CRC Press (1997), for example.
In JP-A-2001-15055, moreover, it is described that the information on the side face of a specimen is obtained by irradiating the specimen obliquely with an electron beam to take an image.